1. Field of the Invention
This invention relates to a camera module, and more specifically to a camera module that can be used in small-sized electronic apparatuses such as digital cameras, camera-equipped cellular phones and the like.
2. Description of the Prior Art
A camera module having a function of displacing a lens unit in a direction of an optical axis of the lens unit is used in relatively small-sized digital cameras and camera-equipped cellular phones and the like. This function is used for providing an autofocus function and/or zoom function and the like, and is achieved by an interaction between a magnetic field generated by an electrical current flowing in a coil and a magnetic field generated by a yoke and magnets provided on the yoke.
Such a camera module includes a barrel which holds the lens unit, a holder which houses the barrel therein, and a pair of leaf springs for supporting the holder so that the holder is displaceable in a direction of an optical axis of the lens unit.
FIG. 11 is an exploded perspective view of an autofocus actuator assembly 100 used in a conventional camera module for displacing a lens unit (not shown) in a direction of an optical axis thereof. It is to be noted that an imaging element (not shown) is provided below a base 111 of the actuator assembly 100.
A holder 108 that houses a barrel (not shown) holding the lens unit is provided between a cover 101 and the base 111 so that the holder 108 is displaceable in a direction of an optical axis of the lens unit together with the lens unit.
An inner annular portion 104b of an upper leaf spring 104 and an inner annular portion 110b of a lower leaf spring 110 are attached to the upper and lower cylindrical portions of the holder 108, respectively. Further, an outer annular portion 104a of the upper leaf spring 104 is attached to an upper surface of a yoke 105 which is mounted on the base 111 and an outer annular portion 11a of the lower leaf spring 110 is attached to the base 111, respectively.
Three bridge portions 104c are coupled between the outer annular portion 104a and the inner annular portion 104b of the upper leaf spring 104. As is the same with the upper leaf spring 104, three bridge portions 110c are coupled between the outer annular portion 11a and the inner annular portion 110b of the lower leaf spring 110. By resilient deformation of the respective three bridge portions 104c and 110c, the holder 108 can be displaced in a direction of an optical axis of the lens unit.
A plurality of magnet pieces 106 are bonded to the yoke 105 so as to produce a magnetic circuit. The yoke 105 has an axial bore 105a for receiving the holder 108. Further, a coil 107 is provided around an outer periphery of the holder 108 so that the coil 7 is disposed in the magnetic field produced by the magnet pieces 106 and the yoke 105 in a state that the holder 108 is received in the axial bore 105a. By supplying a current to the coil 107 to generate a magnetic field, a driving force for displaying the holder 108 in a direction of an optical axis of the lens unit is generated. In this regard, it is to be noted that a component denoted by the reference numeral 103 is a flexible printed circuit board used for supplying a current to the coil 107, a component denoted by the reference numeral 102 is a stopper arranged above the upper surface of the inner annular portion 104a of the upper leaf spring 104, and a component donated by the reference numeral 109 is a plate provided between the lower leaf spring 110 and the bottom surface of the yoke 105.
FIG. 12 is a top plan view of the lower leaf spring 110. In this regard, it is to be noted that the upper leaf spring 104 and the lower leaf spring 110 have the common basic structure excepting that the shapes of the outer annular portions 104a, 110a are different from each other. Therefore, in the following description, an explanation will be made only for the lower leaf spring 110, and an explanation for the upper leaf spring 104 is omitted.
As shown in FIG. 12, the lower leaf spring 110 includes a ring-shaped outer annular portion 110a, a ring-shaped inner annular portion 110b arranged inside the outer annular portion 110a through an annular spacing 110f and having the common axis with the outer annular portion 110a, and three bridge portions 110c coupled between the outer annular portion 110a and the inner annular portion 110b for supporting the inner annular portion 104b with respect to the outer annular portion 104a. Each of the bridge portions 104c has an elongated arc shape and provided in the ring-shaped spacing 110f so as to extend along the inner periphery of the outer annular portion 110a and the outer periphery of the inner annular portion 110b through a predetermined angle.
In this regard, it is to be noted that an actuator assembly similar to the actuator assembly 100 having such upper leaf spring 104 and lower leaf spring 110 described above is disclosed in JP-A-No. 2004-280031, for example.
The upper leaf spring 104 and the lower leaf spring 110 are formed from a thin metal plate made of beryllium copper. The thin metal plate is produced through a metal rolling process and it is then subjected to a punching process to form the leaf springs 104, 110. Such a beryllium copper plate has unevenness in its thickness along a direction through which the metal rolling process has been performed. Due to the unevenness of the thickness of the metal plate, there is a problem in that a load characteristic when it is used as the upper leaf spring 104 and the lower leaf spring 110 is not stable. When the leaf springs 104, 110 do not have such a stable load characteristic, an initial response speed and an amount of an initial displacement of the holder 108 when an electrical current is supplied to the coil 107 are not stable, and thus it becomes difficult to precisely control the position of the holder 108.
Further, a load characteristic of the leaf spring is affected by a heat treatment applied during the manufacturing process of the upper leaf spring 104 and the lower leaf spring 110, it is very difficult to obtain a leaf spring having a predetermined load characteristic.
Further, the plurality of magnet pieces 106 provided on the yoke 105 also have minute variations in their thickness. Due to such minute variations in their thickness, there is a case that a distance between each of the magnet pieces 106 and an outer surface of the coil 107 differs. In such a case, there is a problem in that it is not possible to ensure a stable driving force of the holder 108 within the magnetic circuit due to differences in magnetic forces applied to the coil 107.